What level of quality are you looking for? I have both the Klipsch Pro-media Ultra 5.1 and the Logitech Z-5500 which are both considered the best computer speakers you can buy (the Klipsch are much better)
5.1 is a pain with computer speakers because the wires are usually to short. For most my customers i tell them to get the best 2.1 system they can afford.
As for computer speakers logitech and kilpsch are generally accepted to be the best. I have a set of Logitech Z-5500 that sound good but not nearly as good as my buddies bose system.
The other thing you need to look at is what are you sources and what are you pushing them with. Speakers like the Z-5500's are a waste of money if you pushing them with onboard sound or playing music ripped at 128
Also THX in it's hayday used to be a good standard, but even then it was flawed because it cost companies a small fortune in licensing to get certified, causing many to skip getting certified even though their product meet the standards. Then Lucas sold the THX standard to Creative and it's plummeted since, notice almost all creative products have the THX logo. It's like GM making the h2 and h3 they have nothing to do with the original Humvee, but whoever owns the name can do what they want with it.
Simple answer logitech and kilpsch are the two standards of computer speakers with kilpsch generally being considered the better.
2.1 is better for music all the power is in the two speakers where it is supposed to be, 5.1 speakers will matrix the sound to put it out all the speakers but it doses not sound as good.
5.1 can be helpful in games, but i'm not much of a gamer so i couldn't give you a suggestion either way for that. Play with your sister and dad's computers and see how much sound your getting doing what you would be at home that will help you decide if it's worth the switch.